Objects tend to be particularly vulnerable at their surfaces. The surfaces of objects left in the open bear the brunt of the sun, rain, fog, dew, ice and snow. Under these conditions iron rusts, wood rots and road surfaces crack and disintegrate, just to name a few of the possible consequences. Even sheltered objects, such as those found in the home, suffer the wear of daily use; scratches, dents and abrasions, at their surfaces. To prevent or to minimize such damage, coatings designed to protect surfaces are frequently applied. Coatings can also be used to decorate articles: to add color, luster, or to smooth out roughness or irregularities caused in the manufacturing process. Thus, in selecting a surface coating for a particular object there is a constant balancing which must go on between providing the necessary and appropriate protection and decoration functions. A wide variety of surface coatings is available, e.g., wallpaper, plastic sheets, chrome and silver plating. However, one of the most economical and versatile coatings is paint, which can be applied to any surface, however awkward its size or shape, by one process or another.
Most paints contain liquid resinous or polymeric materials, known as binders. It is this component which, after conversion to a solid through the paint's drying process, provides a surface film having the necessary attributes of adhesion, flexibility, toughness and durability. Paints can generally be subdivided into two broad categories: convertible and non-convertible coatings. A convertible coating is a paint in which the binder is either a polymer precursor, a monomer or a partially polymerized material. Upon addition of a suitable initiator or exposure to radiant energy, such as ultraviolet or infrared radiation, the monomeric or partially polymerized component of the paint undergoes a polymerization reaction in which the binder is converted from a liquid or a soluble state into an insoluble solid material film. Non-convertible coatings, in contrast, do not undergo curing or chemical conversion reactions when they dry. For these paints, film formation involves loss or evaporation of a volatile solvent or dispersion medium and the concomitant deposition of the solid binder material; it is critical that this deposition be both uniform and continuous. See Boxall, et al., Concise Paint Technology, Chemical Publishing, New York, 1977, especially pages 29-57; and Turner, Introduction to Paint Chemistry, Chapman and Hall Ltd., 1967, especially pages 95-107.